The invention relates to brake force controllers. More particularly, the invention relates to a brake force modulator or controller in which there are disposed two hydraulic pistons that are coupled by a lever which is supported on a fulcrum mechanism. The relative position of this fulcrum mechanism is subject to adjustment. The two hydraulic pistons are exposed, respectively, to the brake pressure upstream and downstream of the brake force controller. The load carried by the vehicle is sensed as relative motion of an axle and this motion is transmitted by suitable linkage to the fulcrum mechanism to change its position. In case of malfunction of the linkage, the controller seeks an average, center adjustment. A brake force controller of this type is known from the German Offenlegungsschrift No. 2,435,779. In this known construction, when the automatic control fails, a compromise is made between a "full-load" setting and a "no-load" setting independent of the actual load condition, and the brake force controller then assumes a "middle" setting. Excessive braking of the vehicle due to failure of the linkage is thereby positively prevented.
However, the known brake force controller has the disadvantage that, when the relatively heavy linkage actually breaks, the controller shaft tends to be rotated toward an extreme setting. Such a setting of the controller can result in excessive or insufficient braking of the vehicle.